Systems, methods and transceivers for wireless communications over discontiguous spectrum segments

ABSTRACT

Methods of transmitting a plurality of communications signals over a plurality of discontiguous bandwidth segments in a frequency band include defining a plurality (N FFT ) of orthogonal subcarriers across the frequency band, defining a plurality (N) of available physical subcarriers from among the orthogonal subcarriers. The available physical subcarriers are distributed among the plurality of discontiguous bandwidth segments. The methods further include multiplexing the plurality of communications signals onto the plurality of available physical subcarriers. Multiplexing the plurality of communications signals onto the plurality of available physical subcarriers may include assigning the communications signals to respective ones of a plurality (N) of logical subcarriers, and mapping the plurality of logical subcarriers to corresponding ones of the plurality of available physical subcarriers. Related transmitters, receivers and communications systems are also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 11/679,598filed Feb. 27, 2007 which application claims the benefit of and priorityto U.S. Provisional Patent Application No. 60/777,602, filed Feb. 28,2006, the disclosures of which are hereby incorporated herein byreference as if set forth in their entireties.

FIELD OF THE INVENTION

This invention relates to wireless communications systems and methods,and more particularly to terrestrial and/or satellite wirelesscommunications systems and methods.

BACKGROUND

Wireless communications systems and methods are widely used fortransmitting and/or receiving information between at least two entitiesusing a modulated carrier frequency that occupies a substantiallycontiguous band of frequencies over a predetermined bandwidth. Forexample, an Orthogonal Frequency Division Multiplexed/Multiple Access(OFDM/OFDMA) communications system and method may use a number ofmodulated sub-carriers which are contiguously configured in frequency soas to occupy an aggregate (overall) carrier bandwidth of, for example,1.25 MHz. Terrestrial wireless communications systems and methods may bebased on cellular/PCS and/or other techniques.

Satellite communications systems and methods are based on wirelesscommunications technologies and employ at least one space-basedcomponent, such as one or more satellites, that is/are configured tocommunicate with a plurality of satellite radioterminals. A satelliteradioterminal communications system or method may utilize a singleantenna beam covering an entire area served by the system.Alternatively, in cellular satellite radioterminal communicationssystems and methods, multiple beams are provided, each of which canserve distinct geographical areas in the overall service region, tocollectively serve an overall satellite footprint. Thus, a cellulararchitecture similar to that used in conventional terrestrialcellular/PCS radioterminal systems and methods can be implemented incellular satellite-based systems and methods. The satellite typicallycommunicates with radioterminals over a bidirectional communicationspathway, with radioterminal communication signals being communicatedfrom the satellite to the radioterminal over a downlink or forward link,and from the radioterminal to the satellite over an uplink or returnlink.

As used herein, the term “radioterminal” includes cellular and/orsatellite radioterminals; Personal Communications System (PCS) terminalsthat may combine a radioterminal with data processing, facsimile and/ordata communications capabilities; Personal Digital Assistants (PDA) thatcan include a radio frequency transceiver and a pager, Internet/Intranetaccess, Web browser, organizer, calendar and/or a global positioningsystem (GPS) receiver; and/or conventional laptop and/or palmtopcomputers or other appliances, which include a radio frequencytransceiver. As used herein, the term “radioterminal” also includes anyother radiating user device/equipment/source that may have time-varyingor fixed geographic coordinates, and may be portable, transportable,installed in a vehicle (aeronautical, maritime, or land-based), orsituated and/or configured to operate locally and/or in a distributedfashion at any other location(s) on earth and/or in space. A“radioterminal” also may be referred to herein as a “subscriberstation,” “radiotelephone,” “terminal”, “wireless terminal” or “wirelessuser device”.

Terrestrial networks can enhance cellular satellite radioterminal systemavailability, efficiency and/or economic viability by terrestriallyreusing at least some of the frequency bands that are allocated tocellular satellite radioterminal systems. In particular, it is knownthat it may be difficult for cellular satellite radioterminal systems toreliably serve densely populated areas, because the satellite signal maybe blocked by high-rise structures and/or may not penetrate intobuildings. As a result, the satellite spectrum may be underutilized orunutilized in such areas. The terrestrial reuse of at least some of thesatellite system frequencies can reduce or eliminate this potentialproblem.

Moreover, the capacity of a hybrid system, comprising terrestrial andsatellite-based connectivity and configured to terrestrially reuse atleast some of the satellite-band frequencies, may be higher than acorresponding satellite-only system since terrestrial frequency reusemay be much denser than that of the satellite-only system. In fact,capacity may be enhanced where it may be mostly needed, i.e., in denselypopulated urban/industrial/commercial areas where theconnectivity/signal(s) of a satellite-only system may be unreliable. Asa result, a hybrid (satellite/terrestrial cellular) system that isconfigured to reuse terrestrially at least some of the frequencies ofthe satellite band may become more economically viable, as it may beable to serve more effectively and reliably a larger subscriber base.

U.S. Pat. No. 6,684,057, to Karabinis, and entitled Systems and Methodsfor Terrestrial Reuse of Cellular Satellite Frequency Spectrum, thedisclosure of which is hereby incorporated herein by reference in itsentirety as if set forth fully herein, describes that a satellitefrequency can be reused terrestrially by an ancillary terrestrialnetwork even within the same satellite cell, using interferencecancellation techniques. In particular, a system according to someembodiments of U.S. Pat. No. 6,684,057 includes a space-based componentthat is configured to receive wireless communications from a firstradiotelephone in a satellite footprint over a satellite radiotelephonefrequency band, and an ancillary terrestrial network that is configuredto receive wireless communications from a second radiotelephone in thesatellite footprint over the satellite radiotelephone frequency band.The space-based component also receives the wireless communications fromthe second radiotelephone in the satellite footprint over the satelliteradiotelephone frequency band as interference, along with the wirelesscommunications that are received from the first radiotelephone in thesatellite footprint over the satellite radiotelephone frequency band. Aninterference reducer is responsive to the space-based component and tothe ancillary terrestrial network that is configured to reduce theinterference from the wireless communications that are received by thespace-based component from the first radiotelephone in the satellitefootprint over the satellite radiotelephone frequency band, using thewireless communications that are received by the ancillary terrestrialnetwork from the second radiotelephone in the satellite footprint overthe satellite radiotelephone frequency band.

Satellite radioterminal communications systems and methods that mayemploy terrestrial reuse of satellite frequencies are also described inU.S. Pat. No. 6,785,543 to Karabinis, entitled Filters For CombinedRadiotelephone/GPS Terminals, and Published U.S. Patent Application Nos.US 2003/0054761 to Karabinis, entitled Spatial Guardbands forTerrestrial Reuse of Satellite Frequencies; US 2003/0054814 to Karabiniset al., entitled Systems and Methods for Monitoring Terrestrially ReusedSatellite Frequencies to Reduce Potential Interference; US 2003/0054762to Karabinis, entitled Multi-Band/Multi-Mode Satellite RadiotelephoneCommunications Systems and Methods; US 2003/0153267 to Karabinis,entitled Wireless Communications Systems and Methods UsingSatellite-Linked Remote Terminal Interface Subsystems; US 2003/0224785to Karabinis, entitled Systems and Methods for Reducing Satellite FeederLink Bandwidth/Carriers In Cellular Satellite Systems; US 2002/0041575to Karabinis et al., entitled Coordinated Satellite-TerrestrialFrequency Reuse; US 2002/0090942 to Karabinis et al., entitledIntegrated or Autonomous System and Method of Satellite-TerrestrialFrequency Reuse Using Signal Attenuation and/or Blockage, DynamicAssignment of Frequencies and/or Hysteresis; US 2003/0068978 toKarabinis et al., entitled Space-Based Network Architectures forSatellite Radiotelephone Systems; US 2003/0153308 to Karabinis, entitledStaggered Sectorization for Terrestrial Reuse of Satellite Frequencies;and US 2003/0054815 to Karabinis, entitled Methods and Systems forModifying Satellite Antenna Cell Patterns In Response to TerrestrialReuse of Satellite Frequencies, US 2004/0121727 to Karabinis, entitledSystems and Methods For Terrestrial Reuse of Cellular SatelliteFrequency Spectrum In A Time-Division Duplex Mode, US 2004/0192293 toKarabinis, entitled Aggregate Radiated Power Control ForMulti-Band/Multi-Mode Satellite Radiotelephone Communications SystemsAnd Methods, US 2004/0142660 to Churan, entitled Network-Assisted GlobalPositioning Systems, Methods And Terminals Including Doppler Shift AndCode Phase Estimates, and US 2004/0192395 to Karabinis, entitledCo-Channel Wireless Communication Methods and Systems UsingNonsymmetrical Alphabets, all of which are assigned to the assignee ofthe present invention, the disclosures of all of which are herebyincorporated herein by reference in their entirety as if set forth fullyherein.

Satellite communications systems and methods may be used for voiceand/or data. Moreover, satellite communications systems and methods areincreasingly being used with broadband information, such as multimediainformation. Unfortunately, it may be difficult to send and receivebroadband information over conventional satellite communications systemsand methods. In particular, communications frequencies allocated tosatellite communications may be highly fragmented, and may not includecontiguous segments having a wide enough bandwidth to individuallysupport broadband communications. Moreover, as the demand for widerbandwidth communications systems and methods increases, there may beincreased need to utilize non-contiguous bandwidth segments forcommunication of a broadband communications signal for both satelliteand terrestrial based communications.

Communications systems and methods for transmitting broadband signalsover discontiguous frequency segments are disclosed in commonly assignedand copending U.S. patent application Ser. No. 11/006,318, filed Dec. 7,2004 and entitled “Broadband Wireless Communications Systems and MethodsUsing Multiple Non-Contiguous Frequency Bands/Segments.” As demand forbroadband communications using discontiguous frequency bands increases,improved communications systems and/or methods may be desired.

SUMMARY

Some embodiments of the invention provide methods of transmitting aplurality of communications signals over a plurality of discontiguousbandwidth segments in a frequency band. The methods include defining aplurality (N_(FFT)) of orthogonal subcarriers across the frequency band,and defining a plurality (N) of available physical subcarriers fromamong the orthogonal subcarriers, where N<N_(FFT). The availablephysical subcarriers are distributed among at least some of theplurality of discontiguous bandwidth segments. The methods furtherinclude multiplexing the plurality of communications signals onto theplurality of available physical subcarriers.

Multiplexing the plurality of communications signals onto the pluralityof available physical subcarriers may include assigning thecommunications signals to respective ones of a plurality (N) of logicalsubcarriers, and mapping the plurality of logical subcarriers tocorresponding ones of the plurality of available physical subcarriers.

Multiplexing the communications signals onto the plurality of availablephysical subcarriers may further include assigning at least one pilotsignal to at least one of the plurality of logical subcarriers, andmodulating each of the available physical subcarriers with data from acorresponding logical subcarrier.

The methods may further include defining a plurality (N_(FFT)-N) ofunavailable physical subcarriers within the frequency band, and settingan input data signal corresponding to each of the plurality ofunavailable physical subcarriers to zero. The communications signals andthe at least one pilot signal may include N information signalscorresponding to the available physical subcarriers, and modulating eachof the available physical subcarriers with data from a correspondinglogical subcarrier may include performing an N_(FFT)-point inversefourier transform of the N information signals corresponding to theavailable physical subcarriers and the N_(FFT)-N input data signalscorresponding to each of the plurality of unavailable physicalsubcarriers.

The methods may further include converting an output of theN_(FFT)-point inverse fourier transform to a serial data stream, andtransmitting the serial data stream.

The methods may further include defining a plurality of clusters in thediscontiguous bandwidth segments, each of the clusters including aplurality contiguous subcarriers, defining an adaptive modulation andcoding (AMC) subchannel including a plurality of contiguous clustersextending over a plurality of contiguous symbols, allocating a firstplurality of subcarriers within the AMC subchannel as pilot subcarriers,such that the pilot subcarriers are distributed uniformly across the AMCsubchannel, and allocating a second plurality of subcarriers within theAMC subchannel as data subcarriers.

Defining the AMC subchannel may include defining a subchannel includingtwo clusters over three symbols or one cluster over six symbols. Thepilot subcarriers for an AMC subchannel may be allocated at locationsdetermined by the indices of logical subcarriers, and the pilotsubcarriers in an AMC channel may be offset by two subcarriers inadjacent symbols.

The pilot subcarriers for an AMC channel including nine subcarriers percluster may be determined according to the equation:

pilot_subs(n, k) = 9n + 3m + 1${{{for}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},\left\lceil \frac{N}{9} \right\rceil,$

where n is an index number of a pilot subcarrier in a symbol, k is anindex of the symbol, m is given by k mod 3, and ┌X┘ denotes the largestinteger not greater than X.

The methods may further include assigning a plurality of data symbols todata subcarriers in the AMC subchannel according to a scramblingsequence, such as a scrambling sequence defined by a Galois field and anoffset. The scrambling sequence may be unique to a particular celland/or sector of a wireless communications system.

Multiplexing the plurality of communications signals onto the pluralityof available physical subcarriers may include spreading the plurality ofcommunications signals using a plurality of corresponding spreadingcodes, combining the plurality of spread communications signals to forma combined communications signal, and converting the combinedcommunications signal to parallel communications signals. Combining theplurality of communications signals to form a combined communicationssignal may include combining the plurality of communications signalswith a pilot signal.

Multiplexing the plurality of communications signals onto the pluralityof available physical subcarriers may include assigning the parallelcommunications signals to respective ones of a plurality (N) of logicalsubcarriers, and mapping the plurality of logical subcarriers tocorresponding ones of the plurality of available physical subcarriers.

Multiplexing the communications signals onto the plurality of availablephysical subcarriers may include assigning at least one pilot signal toat least one of the plurality of logical subcarriers, and modulatingeach of the available physical subcarriers with data from acorresponding logical subcarrier.

Some embodiments of the invention provide methods of transmitting aplurality of communications signals over a plurality of discontiguousbandwidth segments in a frequency band, including defining a plurality(N_(FFT)) of orthogonal subcarriers across the frequency band, anddefining a plurality (N) of available physical subcarriers from amongthe orthogonal subcarriers, where the available physical subcarriers aredistributed among the plurality of discontiguous bandwidth segments.

The methods further include receiving M data symbols for each of Kusers, and spreading the data symbols of each user by an L-bit spreadingcode associated with the user to provide M spread data symbols for eachof the K users. The mth data symbols associated with each of the K usersare combined to provide M composite data signals, and the M compositedata signals are converted to parallel input signals having a length L.The M parallel input signals are interleaved to provide Q interleavedinput signals having a length N, and the Q interleaved input signals areassigned to the N available physical subcarriers. The Q interleavedinput signals are transmitted on the N available physical subcarriers.

Transmitting the Q interleaved input signals on the N available physicalsubcarriers may include assigning zeros to N_(FFT)-N unavailablephysical subcarriers to provide N_(FFT) input signals and performing anN_(FFT) point inverse fourier transform on the N_(FFT) input signals.

A transmitter for a wireless communications system according to someembodiments of the invention includes a subcarrier mapper configured toreceive a plurality of input symbols, configured to assign the pluralityof input symbols to N logical subcarriers, configured to map the Nlogical subcarriers to N available physical subcarriers out of N_(FFT)physical subcarriers, and configured to generate N_(FFT) transmitsymbols corresponding to the N_(FFT) physical subcarriers. Thetransmitter further includes an inverse fast fourier transform (IFFT)processor configured to perform an inverse fourier transform on theN_(FFT) transmit symbols output by the subcarrier mapper, and a parallelto serial converter configured to convert an output of the IFFTprocessor to a serial output stream.

The N_(FFT) available physical subcarriers may include orthogonalsubcarriers defined across a frequency band including a plurality ofdiscontiguous available bandwidth segments, and the N available physicalsubcarriers may be distributed among the plurality of discontiguousavailable bandwidth segments.

The subcarrier mapper may be further configured to assign at least onepilot signal to at least one of the plurality of logical subcarriers,and the IFFT processor may be further configured to modulate each of theavailable physical subcarriers with data from a corresponding logicalsubcarrier. The subcarrier mapper may be further configured to set aninput data signal corresponding to each of the plurality of unavailablephysical subcarriers to zero.

The subcarrier mapper may be further configured to define a plurality ofclusters in the discontiguous bandwidth segments, each of the clustersincluding a plurality contiguous subcarriers, configured to define anadaptive modulation and coding (AMC) subchannel including a plurality ofcontiguous clusters extending over a plurality of contiguous symbols,configured to allocate a first plurality of subcarriers within the AMCsubchannel as pilot subcarriers, such that the pilot subcarriers aredistributed uniformly across the AMC subchannel, and configured toallocate a second plurality of subcarriers within the AMC subchannel asdata subcarriers.

The transmitter may further include a plurality of spreaders configuredto spread the communications signals using a plurality of correspondingspreading codes, a combiner configured to combine the plurality ofspread communications signals to form a combined communications signal,and a serial to parallel converter configured to convert the combinedcommunications signal to parallel communications signals. The combinermay be further configured to combine the plurality of communicationssignals with a pilot signal.

The subcarrier mapper may be further configured to assign the parallelcommunications signals to respective ones of a plurality (N) of logicalsubcarriers, and configured to map the plurality of logical subcarriersto corresponding ones of the plurality of available physicalsubcarriers.

The subcarrier mapper may be further configured to assign at least onepilot signal to at least one of the plurality of logical subcarriers,and the IFFT processor may be further configured to modulate each of theavailable physical subcarriers with data from a corresponding logicalsubcarrier.

The subcarrier mapper may be further configured to set an input datasignal corresponding to each of a plurality (N_(FFT)-N) of unavailablephysical subcarriers to zero. The parallel communications signals andthe at least one pilot signal may include N information signalscorresponding to the available physical subcarriers, and the IFFTprocessor may be further configured to perform an N_(FFT)-point inversefourier transform of the N information signals corresponding to theavailable physical subcarriers and the N_(FFT)-N input data signalscorresponding to each of the plurality of unavailable physicalsubcarriers.

The transmitter may further include a parallel to serial converterconfigured to convert an output of the N_(FFT)-point inverse fouriertransform to a serial data stream.

A transmitter for transmitting a plurality of communications signalsover a plurality of discontiguous bandwidth segments in a frequency bandaccording to further embodiments of the invention includes a pluralityof spreaders configured to spread M data symbols for each of K usersaccording to a corresponding spreading code having length L, and aplurality of combiners configured to combine the mth data symbols foreach of the K users to provide M composite spread signals.

The transmitter further includes a plurality of serial to parallelconverters configured to convert the M composite spread signals to Mparallel input signals, and a frequency interleaver configured tointerleave the M parallel input signals to provide Q interleaved inputsignals having a length N. The transmitter further includes a subcarriermapper configured to assign the Q interleaved input signals to the Navailable physical subcarriers, and an inverse fast fourier transform(IFFT) processor configured to modulate N available physical subcarrierswith the Q interleaved input signals.

The subcarrier mapper may be configured to assign zeros to N_(FFT)-Nunavailable physical subcarriers to provide N_(FFT) input signals, andthe IFFT processor may be configured to perform an N_(FFT) point inversefourier transform on the N_(FFT) input signals.

A receiver for a wireless communications system according to someembodiments of the invention includes a serial to parallel converterconfigured to convert a received symbol stream to an N_(FFT) symbol wideparallel received symbol stream, and a fast fourier transform (FFT)processor configured to perform an N_(FFT)-point fourier transform onthe parallel received symbol stream and to generate received symbolscorresponding to N_(FFT) subcarriers. The receiver further includes asubchannel demapper configured to select N symbols of the receivedsymbols corresponding to the N_(FFT) subcarriers, where the N selectedsymbols correspond to N available physical subcarriers out of theN_(FFT) subcarriers, and configured to reconstruct a transmit datastream associated with the receiver from the N selected symbols.

The N_(FFT) available physical subcarriers may include orthogonalsubcarriers defined across a frequency band including a plurality ofdiscontiguous available bandwidth segments, and the N available physicalsubcarriers may be distributed among the plurality of discontiguousavailable bandwidth segments.

The subcarrier demapper may be further configured to receive at leastone pilot signal from at least one of the plurality of availablephysical subcarriers.

The subcarrier demapper may be further configured to receive a pluralityof clusters in the discontiguous bandwidth segments, each of theclusters including a plurality contiguous subcarriers, and configured toextract the at least one pilot signal from an adaptive modulation andcoding (AMC) subchannel including a plurality of contiguous clustersextending over a plurality of contiguous symbols.

The subcarrier demapper may be further configured to extract a pluralityof data symbols from data subcarriers in the AMC subchannel according toa scrambling sequence, such as a scrambling sequence defined by a Galoisfield and an offset. The scrambling sequence may be unique to aparticular cell and/or sector of a wireless communications system.

The receiver may further include a parallel to serial converterconfigured to convert the reconstructed transmit data stream to aparallel communications signal, and a despreader configured to despreadthe parallel communications signals using a spreading code associatedwith the receiver.

A communications system according to some embodiments of the inventionincludes a transmitter and a receiver. The transmitter includes asubcarrier mapper configured to receive a plurality of input symbols,configured to assign the plurality of input symbols to N logicalsubcarriers, configured to map the N logical subcarriers to N availablephysical subcarriers out of N_(FFT) physical subcarriers, and configuredto generate N_(FFT) transmit symbols corresponding to the N_(FFT)physical subcarriers. The transmitter further includes an inverse fastfourier transform (IFFT) processor configured to perform an inversefourier transform on the N_(FFT) transmit symbols output by thesubcarrier mapper, and a parallel to serial converter configured toconvert an output of the IFFT processor to a serial output stream.

The receiver includes a serial to parallel converter configured toconvert a received symbol stream to an N_(FFT) symbol wide parallelreceived symbol stream, and a fast fourier transform (FFT) processorconfigured to perform an N_(FFT)-point fourier transform on the parallelreceived symbol stream and to generate received symbols corresponding toN_(FFT) subcarriers. The receiver further includes a subchannel demapperconfigured to select N symbols of the received symbols corresponding tothe N_(FFT) subcarriers, where the N selected symbols correspond to Navailable physical subcarriers out of the N_(FFT) subcarriers, andconfigured to reconstruct a transmit data stream associated with thereceiver from the N selected symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate certain embodiment(s) of theinvention. In the drawings:

FIG. 1 is a schematic illustration of a discontiguous spectrum andavailable subcarriers;

FIG. 2 is a schematic illustration of a scheme for the logicalsubcarrier allocation according to some embodiments of the invention;

FIG. 3 is a schematic illustration of a structure of an adaptivemodulation and coding (AMC) subchannel according to some embodiments ofthe invention;

FIG. 4 is a schematic illustration of a structure of an AMC subchannelaccording to further embodiments of the invention;

FIG. 5 is a schematic illustration of a subcarrier allocation for an AMCsubchannel according to some embodiments of the invention;

FIG. 6 is a block diagram of OFDMA systems/methods according to someembodiments of the invention for use with a discontiguous spectrum;

FIG. 7 is a schematic illustration of a chip level mapping betweenlogical and physical subcarriers according to some embodiments of theinvention;

FIG. 8 is a block diagram of CDMA systems/methods according to someembodiments of the invention for use with discontiguous spectrum; and

FIG. 9 is a block diagram of CDMA transmit systems/methods according tofurther embodiments of the invention for use with a discontiguousspectrum.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

As will be appreciated by one of skill in the art, the present inventionmay be embodied as a method, data processing system, and/or computerprogram product. Accordingly, the present invention may take the form ofan entirely hardware embodiment, an entirely software embodiment or anembodiment combining software and hardware aspects all generallyreferred to herein as a “circuit” or “module.” Furthermore, the presentinvention may take the form of a computer program product on a computerusable storage medium having computer usable program code embodied inthe medium. Any suitable computer readable medium may be utilizedincluding hard disks, CD ROMs, optical storage devices, a transmissionmedia such as those supporting the Internet or an intranet, or magneticstorage devices.

The present invention is described below with reference to flowchartillustrations and/or block diagrams of methods, systems and computerprogram products according to embodiments of the invention. It will beunderstood that each block of the flowchart illustrations and/or blockdiagrams, and combinations of blocks in the flowchart illustrationsand/or block diagrams, can be implemented by computer programinstructions. These computer program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

These computer program instructions may also be stored in a computerreadable memory that can direct a computer or other programmable dataprocessing apparatus to function in a particular manner, such that theinstructions stored in the computer readable memory produce an articleof manufacture including instruction means which implement thefunction/act specified in the flowchart and/or block diagram block orblocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

It is to be understood that the functions/acts noted in the blocks mayoccur out of the order noted in the operational illustrations. Forexample, two blocks shown in succession may in fact be executedsubstantially concurrently or the blocks may sometimes be executed inthe reverse order, depending upon the functionality/acts involved.Although some of the diagrams include arrows on communication paths toshow a primary direction of communication, it is to be understood thatcommunication may occur in the opposite direction to the depictedarrows.

Conventional wireless communication standards and/or protocols generallypresume the availability of a contiguous spectrum for the entirebandwidth on which the air interface is implemented. This assumption maynot be valid for all broadband wireless communications systems, due, forexample, to the nature of existing and/or future spectrum allocationswhich may divide available frequencies into blocks that may be smallerthan a desired broadband channel bandwidth. To provide more spectralefficiency and/or to improve the scalability of a communications system,some embodiments of the invention provide OFDMA and/or code divisionmultiple access (CDMA) systems that may operate using a discontiguousspectrum over a wide bandwidth. Systems and/or methods according to someembodiments of the invention may improve the usage of a discontiguousspectrum over a wide bandwidth and/or may improve the spectralefficiency of a communications system.

Unlike 2G and/or 3G systems, future broadband wireless communicationsstandards and/or protocols may support several features which mayprovide improved spectral efficiency and/or may provide more flexiblesystem scalability. Such features may include orthogonal frequencydivision multiplexing (OFDM) using a multi carrier waveform,subchannelization, flexible channel sizes, adaptive modulation andcoding (AMC), and/or multiple inputs and multiple outputs (MIMO)transmit & receive diversity with adaptive antenna systems (AAS). AnOFDM system is a multi-carrier (MC) modulation system in which a serialsymbol sequence is converted to parallel symbol sequences, which areused to modulate a plurality of mutually orthogonal sub-carriers throughIFFT/FFT (Inverse Fast Fourier Transform/Fast Fourier Transform)processing. By decomposing the wideband spectrum into a plurality oforthogonal narrow band subcarriers, OFDM may provide excellent spectralefficiency and/or robustness against frequency selective fading and/ormulti-path fading.

An OFDM-based multiple access scheme (Orthogonal Frequency DivisionMultiple Access, or OFDMA) may allow a plurality of users to sharesystem resources in both the frequency domain (through allocation ofsubcarriers) and the time domain. For example, the current WiMaxstandard supports the OFDMA physical layer (PHY) mode with at least oneof the FFT sizes (N_(FFT)) being 2048 (which is backwards compatible to802.16-2004), 1024, 512 and/or 128. Having flexible FFT sizesfacilitates the support of various channel bandwidths. An OFDM symbolincludes several types of subcarriers, which include data subcarriersfor data transmission, pilot subcarriers for various estimationpurposes, and null carriers (no transmission at all) for guard bands andthe DC carrier. The active subcarriers are divided into subsets ofsubcarriers. Each subset of subcarriers is termed a “subchannel.” In thedownlink (i.e. the link from a base station/satellite to aradioterminal), a subchannel may be intended for different groups ofreceivers; in the uplink (i.e. the link from the radioterminal to thebase station/satellite), a transmitter may be assigned one or moresubchannels, and several transmitters may transmit simultaneously. Thesubcarriers forming one subchannel may be either distributed (i.e.discontiguous) and/or adjacent to one another.

In OFDMA, a data region is a two-dimensional (time and frequency)allocation of a group of contiguous subchannels in a group of contiguousOFDM symbols. A data region can be transmitted in the downlink by thebase station/satellite as a transmission to a radioterminal and/or agroup of radioterminals.

In OFDMA systems and/or methods, data is mapped to available subchannelsas follows. First, the data is segmented into blocks sized to fit intoone OFDMA slot, and the slots are mapped such that the lowest numberedslot occupies the lowest numbered subchannel in the lowest numbered OFDMsymbol. As the mapping is continued, the OFDM symbol index is increased.When the edge of the data region is reached, the mapping is continuedfrom the lowest numbered OFDM symbol in the next available subchannel.

Depending on capacity and spectral mask requirements, subchannelallocation in the downlink may be performed using partial usage of thesubchannels (PUSC), in which some of the subchannels are allocated tothe transmitter, and/or full usage of the subchannels (FUSC), in whichall subchannels are allocated to the transmitter. In the uplink, OFDMAsubchannel allocation is typically performed with PUSC.

The OFDMA data is mapped to an OFDMA data region (or a group of slots)for both the downlink and the uplink. A slot in the OFDMA PHY definitionincludes both a time and a subchannel dimension for completeness and isthe minimum possible data allocation unit. The definition of an OFDMAslot depends on the structure, which may vary for the uplink and thedownlink, for FUSC and PUSC, and for the distributed subcarrierpermutations and the adjacent subcarrier permutation.

For downlink FUSC using distributed subcarrier permutation, a slot isone subchannel by one OFDM symbol. For downlink PUSC using distributedsubcarrier permutation, one slot is one subchannel by two OFDM symbols.For uplink PUSC using either of the distributed subcarrier permutations,one slot is one subchannel by three OFDM symbols. For uplink anddownlink using the adjacent subcarrier permutation, one slot is onesubchannel by one OFDM symbol.

The number of orthogonal subcarriers that make up an OFDM symbol isdetermined by the FFT size N_(FFT). Subtracting the guard tones fromN_(FFT), one obtains the set of “used” subcarriers N_(used). For boththe uplink and the downlink, these used subcarriers are allocated to thepilot subcarriers and data subcarriers. However, there is a differencebetween the different possible zones. For FUSC, in the downlink, thepilot tones are allocated first, and the remaining subcarriers aredivided into subchannels that are used exclusively for data. For PUSC inthe downlink or in the uplink, the set of used subcarriers is firstpartitioned into subchannels, and then the pilot subcarriers areallocated from within each subchannel. Thus in FUSC, there is one set ofcommon pilot subcarriers, but in PUSC, each subchannel contains its ownset of pilot subcarriers.

The current WiMax standard always assumes the existence of a continuousspectrum for the entire bandwidth on which the IFFT/FFT processing isperformed. Therefore, the standard specifies the subcarrier allocationsfor both downlink and uplink based on a contiguous set of availablesubcarriers. However, in some cases, such as a situation where acontiguous spectrum block may not be available, it may be desirable totransmit/receive communications signals using a discontiguous spectrumto improve spectral efficiency. A discontiguous spectrum may consist ofa few discontiguous frequency bands in a certain spectrum segment, forexample, the spectrum on the L-Band consisting of a few discontiguousfrequency bands from 1626.5 MHz to 1660.5 MHz with total availablespectrum about 11 MHz. Given such a spectrum, any frequency band that issmaller than 1.25 MHz may not be usable according to current standards,and/or any frequency band whose bandwidth is not an integral multiple of1.25 MHz may not be fully utilized according to current standards.

As for a CDMA system, if the spread spectrum signal includesdiscontiguous frequency segments, the system may suffer performance lossdue to the missing parts of the spectrum and/or due to interference fromsignals in the interstitial frequency band(s). Under the currentcdma2000 standard, any frequency band that has less than 1.25 MHzcontiguous bandwidth may not be used for the 1x system, and anyfrequency band that has less than 3.75 MHz contiguous bandwidth may notbe used for the 3x system.

Thus, in conventional OFDMA and/or CDMA communications systems, aspectrum including discontiguous of frequency bands that are smallerthan 1.25 MHz may not be fully utilized. Some embodiments of the presentinvention provide techniques that may make use of those smallerfrequency bands as part of integral spectrum over which OFDMA and/orCDMA may be implemented. Accordingly, some embodiments of the inventionmay provide more spectral efficiency and/or system scalability forhigher data rate communications in both the downlink and/or the uplinkcommunications channels.

1. An OFDMA System with Discontiguous Spectrum

To operate an OFDMA system over a discontiguous spectrum that consistsof a few frequency bands, there may be the following options for systemdesign: a) operating only on frequency bands that have a continuousbandwidth of at least 1.25 MHz; and b) operating on wider bandwidth thatmay consist of discontinuous spectrum segments that may have a bandwidthless than 1.25 MHz. For Option a), the current WiMax standard may bereadily used. However, using only contiguous 1.25 MHz bandwidth segmentsmay compromise the spectral efficiency and/or scalability of acommunications system. More specifically, portions of the spectrumsmaller than 1.25 MHz may not be usable and therefore may be wasted. Theexclusive use of 1.25 MHz bandwidth segments may also limit the systemthroughput for high-speed access, especially for the downlink. In orderto implement Option (b), however, the systems must operate on adiscontinuous bandwidth, which is not contemplated in the currentstandards.

Some embodiments of the invention provide designs in which thesubcarriers that are used are only those that are contained within theavailable spectrum blocks, after subtracting the guard tones for eachband. All of the subcarriers that belong to unavailable spectrum blocksmay be tuned to zeros. A system/method according to some embodiments ofthe invention may have a high spectral efficiency, in that it may allowefficient use of portions of frequency bands that may otherwise bewasted. Having a wider bandwidth operation may also provide moreflexibility for throughput, which may potentially result in higher datarate access in both the downlink and the uplink.

The formation of a subchannel in an OFDMA system typically involves atwo-dimensional (frequency-time) distributed permutation. As usedherein, the term “permutation” refers to a distribution and/orarrangement of pilot signals and data signals in an AMC subchannel insuch a manner that a receiver may use the pilot signals to accuratelyestimate channels associated with the subcarriers. However, thedistributed subcarrier permutation may only be suitable for a contiguousspectrum, because the data and/or pilot subcarriers that form asubchannel may be uniformly distributed through the entire band. For adiscontiguous spectrum, the distributed subcarrier permutation may nolonger be valid, because some of the potential subcarriers may not beavailable. Accordingly, some embodiments of the invention provide newsubcarrier permutation schemes for use with a discontiguous spectrum.

Referring to FIG. 1, to illustrate an OFDMA system employed over adiscontiguous spectrum, as an example, a frequency bandwidth of BW isshown that consists of a few discontiguous spectrum segments B₁, B₂, . .. , and B₇ separated by interstitial segments I1-I6 that are notavailable to the system. The OFDMA system is assumed to have anN_(FFT)-point IFFT/FFT over the entire bandwidth of BW. The subcarrierspacing is Δf=F_(s)/N_(FFT) where F_(s) is sampling frequency given byfloor(8/7·BW/8000)×8000. The subcarriers falling in the unavailablespectral segments within BW may be turned off (i.e. set to zero). Tolimit out-of-band emission for each spectral segment, a few guardsubcarriers at both sides of each spectrum segment may be set to zero.After setting all the unusable subcarriers to zero, the usablesubcarriers in each spectrum segment include N₁ subcarriers in SegmentB₁, N₂ subcarriers in Segment B₂, N₃ subcarriers in Segment B₃, N₄subcarriers in Segment B₄, N₅ subcarriers in Segment B₅, N₆ subcarriersin Segment B₆, and N₇ subcarriers in Segment B₇. Therefore, the totalnumber of usable subcarriers is given by N=N₁+N₂+ . . . +N₇ afterturning off all of the guard subcarriers. The Nusable physicalsubcarriers are then mapped to N logical subcarriers. Moreover, sincesome of the potential physical subcarriers are not used, in generalN<N_(FFT).

Due to the lack of available subcarriers between two adjacent segments,a subchannel allocation according to some embodiments of the inventionmay be based on an adjacent subcarrier permutation within a segment. Theadjacent subcarrier permutation may apply to both the uplink and thedownlink symbols. Symbol data within a subchannel may be assigned toadjacent subcarriers within a spectrum segment, and the data and pilotsubcarriers may be assigned fixed positions in the frequency domain inan OFDM symbol.

FIG. 2 shows a scheme for the logical allocation of subcarriersaccording to some embodiments of the invention. As used herein, a basicOFDMA allocation unit is called cluster, which is a set of contiguoussubcarriers within a spectrum segment in an OFDM symbol. As shown inFIG. 2, a cluster 10 is a set of 9 contiguous subcarriers within aspectrum segment. In some embodiments, a group of 2 logical consecutiveclusters 10 in an OFDM symbol is defined as a band. In the subchannelallocation, 6 contiguous clusters in a same band are grouped to form anAMC (Adaptive Modulation and Coding) subchannel. FIG. 3 shows thestructures of a band 20 and an AMC subchannel 30 according to someembodiments of the invention.

In some embodiments, a typical AMC subchannel 30 includes 2 clusters ineach of three consecutive OFDM symbols with 6 pilot subcarriers and 48data subcarriers. In other embodiments, a band 20 is the same as acluster 10 in an OFDM symbol. To assign 6 contiguous clusters 10 in asame band 20 to form an AMC subchannel 30, a subchannel consisting of 1cluster in each of six consecutive OFDM symbols may be defined as shownin FIG. 4.

The pilot signals are transmitted on the subchannel band to allow a basestation/satellite to estimate the channel. Because of the discontiguousspectrum, the adjacent subcarrier permutation allows radioterminals toeffectively estimate the channel response and/or a performance metric,such as the carrier-to-interference and noise ratio (CINR), of theassigned subchannel.

It may be desirable to allocate pilot subcarriers such that the pilotsubcarriers are uniformly distributed in an AMC subchannel. Accordingly,in some embodiments, the pilot subcarriers for an AMC subchannel may beallocated at locations determined by the indices of logical subcarriersin an OFDM symbol as follows:

$\begin{matrix}{{{{pilot\_ subs}\left( {n,k} \right)} = {{9n} + {3m} + 1}}{{{{for}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},\left\lceil \frac{N}{9} \right\rceil,}} & (1)\end{matrix}$

where n is the index number of a pilot subcarrier in an OFDM symbol

N is the total number of logical subcarriers,

k is the index of the OFDM symbol

m is given by k mod 3

┌X┘ denotes the largest integer not greater than X

FIG. 5 shows the locations of the pilot subcarriers within an AMCsubchannel according to the allocation scheme described above. As seenin FIG. 5, the pilot subcarriers may be uniformly distributed among theavailable logical subcarriers in an OFDM symbol, and may be offset by 2subcarriers between adjacent OFDM symbols, which may make the channelestimates more efficient. The remaining subcarriers are allocated fordata.

With the adjacent subcarrier AMC subchannel allocation described above,the OFDMA system may be able to detect the CINR of each pilotsubcarrier, and then interpolate the CINRs of the pilot subcarriers toobtain estimates of the CINRs of the data subcarriers in each subchannelThe radioterminal may feed back the CINRs of the subcarriers to the basestation/satellite, and the base station/satellite, using adaptivemodulation and coding (AMC), may assign an AMC subchannel having thebest average CINR for the radioterminal and/or may assign a propermodulation and coding scheme (MCS) based on the CINRs. Since the AMCsubchannel scheme is based on the two dimensional (frequency-time)subchannel allocations, transmission capacity to multiple users in acell may be increased. The adaptive assignment of an AMC subchannelaccording to the channel state of a user may allow service provisioningin compliance with the characteristics of the radioterminal. Also, theadjacent subcarrier permutation for a subchannel may facilitate theimplementation of an adaptive antenna system (AAS) on a per subchannelbasis.

For a terrestrial cellular OFDMA communications system with a frequencyreuse ratio of 1, it may be desirable to consider both the cell-specificidentification and the minimization of co-channel interference in amulti-cell subchannel allocation. In particular, it may be desirable todiscriminate between pilot signals from each cell. To accomplish this,for example, the pilot subcarriers may be spread using orthogonalpseudonoise (PN) codes that are unique for each cell. Thus aradioterminal may demodulate the pilot signal with the PN code specificto its serving cell (or base station/satellite), thereby obtaining anestimate of the channel response and/or CINR of each subcarrier. For anAMC subchannel, the location of data subcarriers may coincide with otherAMC subchannels (in other cells) for the same band. In that case, aradioterminal may fail to decode the desired signal even with areasonable signal to interference plus noise ratio (SINR) if the twocells use the same AMC scheme. Thus, it may be desirable for each cellto have a unique AMC assignment scheme such that the assignment ofsubcarriers is different for each cell.

To address this issue, in some embodiments of the invention, the orderof subcarrier mapping within an AMC subchannel may be scrambled with asequence that is unique to each cell, to thereby provide a systematicpermutation of the subcarrier assignments in each cell/sector. Byscrambling the subcarrier mapping sequences, the number of collisionsbetween any two cells may be reduced. A cell and a subchannel for thecell may be identified through the permutation of a basic sequencedefined in Galois Field (7²), i.e., GF(49) and an offset. The basicGF(49) sequence is based on the prime polynomial as follows:

p(x)=x ²+2x+3  (2)

The basic sequence consists of the field elements of GF(7²) that can bewritten as

Q={α,α², . . . , α⁴⁸}  (3)

and their polynomial representation is given by the remainder of x″ upondivision by the prime polynomial p(x):

$\begin{matrix}{{\alpha^{n} = {{Remainder}\left\{ \frac{x^{n}}{p(x)} \right\}}},{n = 1},2,\ldots \mspace{14mu},48} & (4)\end{matrix}$

The field elements of GF(49) are derived with different representations,and the results are shown in Table 1.

TABLE 1 The GF(49) Field Elements Generated Using the Prime Polynomialin (x) Exponential Polynomial Hepta Decimal Represent. Represent.Represent Represent a¹  x 10 7 a² 5x + 4 54 39 a³  x + 6 16 13 a⁴ 4x + 444 32 a⁵ 3x + 2 32 23 a⁶ 3x + 5 35 26 A⁷ 6x + 5 65 47 a⁸ 3 03 3 a⁹  3x30 21 a¹⁰  x + 5 15 12 a¹¹ 3x + 4 34 25 a¹² 2x + 2 22 16 a¹³ 2x + 6 2620 a¹⁴ 2x + 1 21 15 a¹⁵ 4x + 1 41 29 a¹⁶ 2 02 2 a¹⁷  2x 20 14 a¹⁸ 3x + 131 22 a¹⁹ 2x + 5 25 19 a²⁰  x + 1 11 8 a²¹ 6x + 4 64 46 a²² 6x + 3 63 45a²³ 5x + 3 53 38 a²⁴ 6 06 6 a²⁵  6x 60 42 a²⁶ 2x + 3 23 17 a²⁷ 6x + 1 6143 a²⁸ 3x + 3 33 24 a²⁹ 4x + 5 45 33 a³⁰ 4x + 2 42 30 a³¹  x + 2 12 9a³² 4 04 4 a³³  4x 40 28 a³⁴ 6x + 2 62 44 a³⁵ 4x + 3 43 31 a³⁶ 5x + 5 5540 a³⁷ 5x + 1 51 36 a³⁸ 5x + 6 56 41 a³⁹ 3x + 6 36 27 a⁴⁰ 5 05 5 a⁴¹  5x50 35 a⁴² 4x + 6 46 34 a⁴³ 5x + 2 52 37 a⁴⁴ 6x + 6 66 48 a⁴⁵  x + 3 1310 a⁴⁶  x + 4 14 11 a⁴⁷ 2x + 4 24 18 a⁴⁸ 1 01 1

The GF(49) permutation basic sequence may be expressed in decimalrepresentation as

Q={7,39,13,32,23,26,47,3,21,12,25,16,20,15,29,2,14,22,19,8,46,45,38,6,42,17,43,24,33,30,9,4,28,44,31,40,36,41,27,5,35,34,37,48,10,11,18,1}  (5)

In an AMC subchannel, after mapping the 6 pilot subcarriers (i.e. oneper cluster of the subchannel), the remaining 48 subcarriers (i.e. 8 percluster) may be used for the traffic data. As shown in FIG. 5, within anAMC subchannel, the traffic data subcarriers are indexed from 1 to 48starting with the first in the first cluster, and increasing along thesubcarriers first, then by cluster. To reduce potential co-channelinterference and/or to enable the identification of a cell in amulti-cell environment with a frequency reuse ratio of 1, the datasubcarriers of an AMC channel may be permutated using a permutation basesequence such as the permutation sequence Q with cell identification.For this permutation scheme, the data subcarrier for the k^(th) user inthe n^(th) Cell may be mapped to the subcarrier of a subchannelaccording to the following equation:

$\begin{matrix}{{{subcarrier}\left( {k,n} \right)} = \left\{ \begin{matrix}\begin{matrix}{{Q_{m}(k)} +} \\{\left( \left\lceil \frac{n}{48} \right\rceil \right){mod}\; 49}\end{matrix} & {{{if}\mspace{14mu} \begin{Bmatrix}{{Q_{m}(k)} +} \\{\left( \left\lceil \frac{n}{48} \right\rceil \right){mod}\; 49}\end{Bmatrix}{mod}\; 49} \neq 0} \\{\left( \left\lceil \frac{n}{48} \right\rceil \right){mod}\; 49} & {{{if}\mspace{14mu} \begin{Bmatrix}{{Q_{m}(k)} +} \\{\left( \left\lceil \frac{n}{48} \right\rceil \right){mod}\; 49}\end{Bmatrix}{mod}\; 49} = 0}\end{matrix} \right.} & (6)\end{matrix}$

wherem=n mod 48Q_(m)(k)=the k^(th) element of the left cyclic shifted version ofpermutation base sequence Q by mX mod Y=remainder of X/Y, and┌X┘ denotes largest integer not greater than X.

FIG. 6 shows a block diagram for OFDMA systems and/or methods includingan OFDM transmitter 100 and an OFDM receiver 200 configured tocommunicate over a discontiguous spectrum according to some embodimentsof the invention.

As shown in FIG. 6, in a transmitter 100, data is divided into Ksubchannels which are individually subjected to forward error correctionin forward error correction blocks 110, interleaving via bitinterleavers 115, and subchannel modulation in subchannel modulationblocks 120. The modulated subchannel signals are provided to asubchannel bank mapper 130 which is configured to map logical subchanneldata and pilot signals generated by a pilot signal generator 125 tophysical subchannels using cell identification numbers.

The cell identification number may be related to a specific base stationin a terrestrial OFDMA cellular system or a particular spot-beam in amultiple beam satellite OFDMA system.

The mapped subchannel streams are then processed according toconventional OFDM methods. Namely, the mapped subchannel streams areprocessed by an Inverse Fast Fourier Transform (IFFT) block 140 andconverted to a serial stream by a parallel to serial conversion block145. A cyclic extension is then appended to the serialized stream by acyclic extension block 150 in order to mitigate inter-symbolinterference, and the resulting pulse stream is processed by a pulseshaping filter 155. The pulse stream is then converted to analog via adigital to analog converter 160 and transmitted by transmitter 165.

In a receiver 200, the transmitted waveform is received by an RFreceiver front end 210, converted to digital by an analog to digitalconverter 215, and filtered by a match filter 220. The cyclic extensionis stripped from the data stream by a cyclic extension removal block225.

The resulting data stream is converted to parallel by a serial toparallel converter 230 and processed by an Fast Fourier Transformer(FFT) 235, according to conventional OFDM processing techniques.

The resulting subchannel information is then demultiplexed and de-mappedinto logical subchannel information by a subchanneldemultiplexer/de-mapper 240 utilizing cell identification numbers aswell as subchannel indices. The resulting reconstructed data stream isthen the demodulated via a demodulator block 245, de-interleaved by adeinterleaver 250 and decoded by a forward error correction decoder 255to reconstruct the transmitted data.

A permutation scheme according to embodiments of the invention maysystematically provide not only a means of identifying a particular basestation or spot-beam in an OFDMA communications system, but also methodsfor reducing co-channel interference between subchannels of differentbase stations and/or spot-beams in an OFDMA communications system with afrequency reuse ratio of 1. The scheme may be extended to a cellularsystem with multi-sector cells by applying the permutation method toeach sector having the same frequency band among different cells.

2. A CDMA System with Discontiguous Spectrum

A direct sequence (DS) spread spectrum CDMA system typically operatesusing a contiguous spectrum due to the nature of bandwidth spreading ina CDMA system. Otherwise, the performance of the DS-CDMA system may becompromised because the missing spectrum may result in partial loss ofthe CDMA signal and/or interference from some interstitial frequencies.However, according to some embodiments of the invention, instead ofapplying spreading sequences in the time domain as is customary,spreading sequences may be applied in the frequency domain by mappingdifferent chips of a spreading sequence to individual OFDM subcarriersover discontiguous bandwidth segments. Such a system may be referred toas a discontiguous multi-carrier (MC) CDMA system. In a discontiguousMC-CDMA system according to some embodiments of the invention, each OFDMsubcarrier may have a data rate that is the same as the original inputdata rate, and the chip data may be distributed across multiplesubcarriers. Since different chips of a spreading sequence are mapped toindividual OFDM subcarriers, it is possible according to someembodiments of the invention to send a CDMA signal through adiscontiguous spectrum by using only those subcarriers that areavailable to the system.

Generally, in an MC-CDMA system according to embodiments of theinvention, the transmitted signal of the i^(th) data symbol of thek^(th) user, as a function of time, may be written as

$\begin{matrix}\begin{matrix}{{y_{i}^{k}(t)} = {\sum\limits_{n = 0}^{N_{FFT} - 1}{b_{i}^{k}q_{n}^{k}^{{{j2\pi}{({f_{0} + {n\; \Delta \; f}})}}t}{p\left( {t - {\; t}} \right)}}}} \\{= {\sum\limits_{n = 0}^{N_{FFT} - 1}{{s_{i}^{k}(n)}^{{{j2\pi}{({f_{0} + {n\; \Delta \; f}})}}t}{p\left( {t - {\; T}} \right)}}}}\end{matrix} & (7)\end{matrix}$

where

N_(FFT) is the number of IFFT/FFT points, i.e., the number ofsubcarriers available in the entire bandwidth BW

b_(i) ^(k) is the i^(th) data symbol of the k^(th) user

f_(o) is the lowest subcarrier frequency

q_(n) ^(k) is the n^(th) element of the sequence of N_(FFT) elementsthat is related to the spreading sequence of the k^(th) user

Δf is the subcarrier spacing given by 1/T

s_(i) ^(k) (n)=b_(i) ^(k)q_(n) ^(k), is the subcarrier data sequence,and

${p(t)} \equiv \left\{ \begin{matrix}1 & {{{for}\mspace{14mu} 0} \leq t \leq T} \\0 & {otherwise}\end{matrix} \right.$

With a discontiguous spectrum, some of the N_(FFT) subcarriers may notbe available, as shown in FIG. 1. The unavailable subcarriers may beturned off or set to zero. The remaining subcarriers that fall insideavailable frequency segments are mapped to the logical subcarrierssub_L(n), n=0, 1, . . . , N−1, N<N_(FFT). The logical subcarriers may beused to transmit the chip level signal. For example, if the number ofchips per symbol is equal to N, then the chip level signal for thei^(th) symbol of the k^(th) user may be represented by

w _(i) ^(k)(n)=b _(i) ^(k) c _(n) ^(k) , n=0, 1, . . . , N−1,  (8)

where c_(n) ^(k) is the n^(th) chip of the spreading sequence of thek^(th) user.

The chip level signal may be mapped to the logical subcarriers one byone as follows:

sub _(—) L(n)=w _(i) ^(k)(n), n=0, 1, . . . , N−1  (9)

The mapping of logical subcarriers to physical subcarriers may beperformed based on the availability of physical subcarriers, afterexcluding the unavailable subcarriers and those falling in guard-bands.The excluded physical subcarriers may be padded with zeros. Then thelogical subcarriers may be mapped to the remaining physical subcarriersone by one in order from the lowest available physical subcarrier to thehighest available physical subcarrier. FIG. 7 illustrates a mappingprocedure in which one CDMA data symbol is mapped to one OFDM symbol(e.g., when the number of chips in the spreading sequence is equal tothe number of available physical subcarriers). As a result, the physicalsubcarrier data sequence s_(i) ^(k)(n) in Equation (7) is equal to themapped physical subcarrier sequence as shown in FIG. 7. If the indicesof the available physical subcarriers are given by ps_index(m), n=0, 1,. . . , N−1, which are integers in the range of (0, N_(FFT)−1), then themapped physical subcarrier sequence s_(i) ^(k)(n), n=0, 1, . . . ,N_(FFT)−1 may be written as

$\begin{matrix}{{s_{i}^{k}(n)} = \left\{ \begin{matrix}{w_{i}^{k}(m)} & {{{{for}\mspace{14mu} n} = {{ps\_ index}(m)}},{m = 0},1,\ldots \mspace{14mu},{N - 1}} \\0 & {otherwise}\end{matrix} \right.} & (10)\end{matrix}$

With the mapped physical subcarrier sequence and Equation (7), thetransmitted signal of the i^(th) data symbol of the k^(th) user may bewritten as

$\begin{matrix}\begin{matrix}{{y_{i}^{k}(t)} = {\sum\limits_{n = 0}^{N_{FFT} - 1}{{s_{i}^{k}(n)}^{{{j2\pi}{({f_{0} + {n\; \Delta \; f}})}}t}{p\left( {t - {\; T}} \right)}}}} \\{= {\sum\limits_{m = 0}^{N - 1}{{w_{i}^{k}(m)}^{{j2}\; {\pi {({f_{0} + {p\; s\; \_ \; {{index}{(m)}}\Delta \; f}})}}t}{p\left( {t - {\; T}} \right)}}}}\end{matrix} & (11)\end{matrix}$

For a system with total of K users, the transmitted signal is acomposite superposition of all chip level subcarrier sequences. Theunique spreading sequence c_(n) ^(k)=0,1, . . . , N−1 separates otherusers from the k^(th) user, provided that the spreading sequences of theusers are orthogonal to each other.

FIG. 8 is a block diagram illustrating systems and/or methods accordingto some embodiments of the invention. Referring to FIG. 8, a transmitter300 configured to transmit a data signal using discontiguous MC-CDMA isillustrated. As shown therein, K users' traffic data is spread usinguser-specific long spreading codes via spreaders 310. The resultingsignals are combined along with a pilot signal in a combiner 315, andthe combined signal is spread with a covering code unique to the cellvia a spreader 320.

The resulting data stream is converted to parallel signals via a serialto parallel converter 340, and the parallel data streams are mapped tophysical subcarriers via a logical to physical subcarrier mapper 350.Unavailable subcarriers and guardband subcarriers are padded with zerosby a zero padding block 345 using indices of unavailable subcarriers.The resulting physical subcarrier mapping is processed by an IFFT 360and converted to a serial data stream by a parallel to serial converter370, according to conventional OFDM processing techniques. Cyclicextension, pulse shaping, digital to analog conversion and/ortransmission may then be performed on the serial data stream by atransmitter block 380.

In a receiver 400, as shown in FIG. 8, the transmitted signal isreceived via a receiver front end 410 which may also performanalog-to-digital conversion, and match filtering. The transmittedsubcarriers in an OFDM symbol are recovered using an FFT operation in anFFT block 430 after sampling the received signal at the chip rate andconverting the received signal to parallel via a serial to parallelconverter 420. The recovered physical subcarriers are then converted tothe logical subcarrier sequence in a physical to logical subcarrierdemapping block 440. The resulting signal, which corresponds to the chiplevel signal in a symbol is then converted to a serial data stream by aparallel to serial converter 450. After despreading the chip levelsignal via a despreader 460, the user symbol level data is recovered bya demodulator 470. The received m^(th) logical subcarrier of the i^(th)symbol may be expressed as

$\begin{matrix}\begin{matrix}{r_{m,j} = {{\sum\limits_{k = 0}^{K - 1}{H_{m}{w_{i}^{k}(m)}}} + n_{m,i}}} \\{= {{\sum\limits_{k = 0}^{K - 1}{H_{m}b_{i}^{k}c_{m}^{k}}} + n_{m,i}}}\end{matrix} & (12)\end{matrix}$

where H_(m) is the channel frequency response of the m^(th) logicalsubcarrier, K is the total number of users, and n_(m,i) is thecorresponding noise term. To recover the k^(th) user data symbol, theMC-CDMA receiver may de-spread the received chip level signal bymultiplying r_(m,i) in Equation (12) by the k^(th) user's spreadingsequence c_(m) ^(k) and by the reciprocal of the channel estimate ofsubcarrier m, m=0, 1, . . . , N−1. The resulting products are summed.Therefore, the decision statistic for the k^(th) user in the i^(th)symbol is given by

$\begin{matrix}{{\hat{d}}_{i}^{k} = {\sum\limits_{m = 0}^{N - 1}{c_{m}^{k}{\hat{\alpha}}_{m}r_{m,i}}}} & (13)\end{matrix}$

where {circumflex over (α)}_(m) is the reciprocal of the channelestimate of subcarrier m, which may be obtained by using the pilotsignal.

In the foregoing example, the number of logical subcarriers N is assumedto be equal to the spreading factor. However, that may not always be thecase. For example, in some cases, the number of logical subcarriers Nmay be larger than the spreading factor L. In that case, a modifiedsystem may offer more flexibility in the system design.

FIG. 9 shows a modified system that may take several data symbols peruser in parallel. The transmit system forms M groups of data symbolsfrom K users, where the first group consists of the first data symbol ofeach user, and the second group consists of the second data symbol ofeach user, and so on. Thus M data symbols of K users are considered at atime. As shown in FIG. 9, the data symbols in each group are spread bytheir corresponding spreading sequences via spreaders 505 and then areadded up by combiners 510. The spreading sequence c^(k) for the k^(th)user is defined as c^(k)=[c₀ ^(k) c₁ ^(k) . . . c_(L-1) ^(k)]. Thesummation of the chip level sequences is series-to-parallel convertedvia serial to parallel converters 520, and sent to a frequencyinterleaver 530 in parallel along with those from other groups. Thefrequency interleaver transforms the M×L subcarrier data matrix to an Nx Q (where Q is an integer) subcarrier data matrix. The interleavedsubcarrier data matrix is then mapped to N logical subcarriers persymbol of Q OFDM symbols in logical to physical subcarrier mapping block540. The logical subcarriers in an OFDM symbol are mapped to physicalsubcarriers in the manner discussed previously. The frequencyinterleaver may also provide frequency diversity for a user bydistributing subcarriers across an entire frequency bandwidth. Theresulting subcarrier data are processed though an IFFT 550 and convertedto serial in a parallel to serial converter 560 to form the OFDM signal.

As noted above, the frequency interleaver transforms the M×L subcarrierdata matrix to an N×Q subcarrier data matrix. Thus, as an example, ifthere are 48 available physical subcarriers (N=48), 12 symbols (M=12)taken from each of K users, and the spreading factor is 16 (L=16), thenM×L=192. Thus, the Q dimension equals 192/48, or 4. Thus, the frequencyinterleaver 530 forms a subcarrier matrix having dimensions 48×4. Thus,4 OFDM symbols may be used to encode 12 symbols from K different users.

In the drawings and specification, there have been disclosed typicalembodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

1. A method of transmitting a plurality of communications signals over aplurality of discontiguous bandwidth segments in a frequency band,comprising: defining a plurality (N_(FFT)) of orthogonal subcarriersacross the frequency band; defining a plurality (N) of availablephysical subcarriers from among the orthogonal subcarriers, wherein theavailable physical subcarriers are distributed among the plurality ofdiscontiguous bandwidth segments; receiving M data symbols for each of Kusers; spreading the data symbols of each user by an L-bit spreadingcode associated with the user to provide M spread data symbols for eachof the K users; combining mth data symbols associated with each of the Kusers to provide M composite data signals; converting the M compositedata signals to parallel input signals having a length L; interleavingthe M parallel input signals to provide Q interleaved input signalshaving a length N; assigning the Q interleaved input signals to the Navailable physical subcarriers; and transmitting the Q interleaved inputsignals on the N available physical subcarriers.
 2. The method of claim1, wherein transmitting the Q interleaved input signals on the Navailable physical subcarriers comprises assigning zeros to N_(FFT)-Nunavailable physical subcarriers to provide N_(FFT) input signals andperforming an N_(FFT) point inverse fourier transform on the N_(FFT)input signals.
 3. A transmitter for a wireless communications system,comprising: a subcarrier mapper configured to receive a plurality ofinput symbols, configured to assign the plurality of input symbols to Nlogical subcarriers, configured to map the N logical subcarriers to Navailable physical subcarriers out of N_(FFT) physical subcarriers, andconfigured to generate N_(FFT) transmit symbols corresponding to theN_(FFT) physical subcarriers; an inverse fast fourier transform (IFFT)processor configured to perform an inverse fourier transform on theN_(FFT) transmit symbols output by the subcarrier mapper; and a parallelto serial converter configured to convert an output of the IFFTprocessor to a serial output stream.
 4. The transmitter of claim 3,wherein the N_(FFT) available physical subcarriers comprise orthogonalsubcarriers defined across a frequency band including a plurality ofdiscontiguous available bandwidth segments, and wherein the N availablephysical subcarriers are distributed among the plurality ofdiscontiguous available bandwidth segments.
 5. The transmitter of claim4, wherein the subcarrier mapper is further configured to assign atleast one pilot signal to at least one of the plurality of logicalsubcarriers; and wherein the IFFT processor is further configured tomodulate each of the available physical subcarriers with data from acorresponding logical subcarrier.
 6. The transmitter of claim 5, whereinthe subcarrier mapper is further configured to set an input data signalcorresponding to each of the plurality of unavailable physicalsubcarriers to zero.
 7. The transmitter of claim 6, wherein thesubcarrier mapper is further configured to define a plurality ofclusters in the discontiguous bandwidth segments, each of the clusterscomprising a plurality contiguous subcarriers, configured to define anadaptive modulation and coding (AMC) subchannel comprising a pluralityof contiguous clusters extending over a plurality of contiguous symbols,configured to allocate a first plurality of subcarriers within the AMCsubchannel as pilot subcarriers, such that the pilot subcarriers aredistributed uniformly across the AMC subchannel, and configured toallocate a second plurality of subcarriers within the AMC subchannel asdata subcarriers.
 8. The transmitter of claim 7, wherein the AMCsubchannel comprises two clusters over three symbols.
 9. The transmitterof claim 7, wherein the AMC subchannel comprises one cluster over sixsymbols.
 10. The transmitter of claim 7, wherein the subcarrier mapperis configured to allocate the pilot subcarriers for an AMC subchannel atlocations determined by the indices of logical subcarriers.
 11. Thetransmitter of claim 7, wherein pilot subcarriers in an AMC channel areoffset by two subcarriers in adjacent symbols.
 12. The transmitter ofclaim 11, wherein the pilot subcarriers for an AMC channel includingnine subcarriers per cluster are determined according to the equation:pilot_subs(n, k) = 9n + 3m + 1${{{for}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},\left\lceil \frac{N}{9} \right\rceil,$wherein: n is an index number of a pilot subcarrier in a symbol; k is anindex of the symbol; m is given by k mod 3; and ┌X┘ denotes the largestinteger not greater than X.
 13. The transmitter of claim 7, wherein thesubcarrier mapper is further configured to assign a plurality of datasymbols to data subcarriers in the AMC subchannel according to ascrambling sequence.
 14. The transmitter of claim 13, wherein thescrambling sequence is defined by a Galois field and an offset.
 15. Thetransmitter of claim 13, wherein the scrambling sequence is unique to aparticular cell and/or sector of a wireless communications system. 16.The transmitter of claim 4, further comprising: a plurality of spreadersconfigured to spread the communications signals using a plurality ofcorresponding spreading codes; a combiner configured to combine theplurality of spread communications signals to form a combinedcommunications signal; and a serial to parallel converter configured toconvert the combined communications signal to parallel communicationssignals.
 17. The transmitter of claim 16, wherein the combiner isfurther configured to combine the plurality of communications signalswith a pilot signal.
 18. The transmitter of claim 16, wherein thesubcarrier mapper is further configured to assign the parallelcommunications signals to respective ones of a plurality (N) of logicalsubcarriers, and configured to map the plurality of logical subcarriersto corresponding ones of the plurality of available physicalsubcarriers.
 19. The transmitter of claim 3, wherein the subcarriermapper is further configured to assign at least one pilot signal to atleast one of the plurality of logical subcarriers, and wherein the IFFTprocessor is further configured to modulate each of the availablephysical subcarriers with data from a corresponding logical subcarrier.20. The transmitter of claim 19, wherein the subcarrier mapper isfurther configured to set an input data signal corresponding to each ofa plurality (N_(FFT)-N) of unavailable physical subcarriers to zero,wherein the parallel communications signals and the at least one pilotsignal comprise N information signals corresponding to the availablephysical subcarriers; and wherein the IFFT processor is furtherconfigured to perform an N_(FFT)-point inverse fourier transform of theN information signals corresponding to the available physicalsubcarriers and the N_(FFT)-N input data signals corresponding to eachof the plurality of unavailable physical subcarriers.
 21. Thetransmitter of claim 20, further comprising a parallel to serialconverter configured to convert an output of the N_(FFT)-point inversefourier transform to a serial data stream.
 22. A transmitter fortransmitting a plurality of communications signals over a plurality ofdiscontiguous bandwidth segments in a frequency band, comprising: aplurality of spreaders configured to spread M data symbols for each of Kusers according to a corresponding spreading code having length L; aplurality of combiners configured to combine the mth data symbols foreach of the K users to provide M composite spread signals; a pluralityof serial to parallel converters configured to convert the M compositespread signals to M parallel input signals; a frequency interleaverconfigured to interleave the M parallel input signals to provide Qinterleaved input signals having a length N; a subcarrier mapperconfigured to assign the Q interleaved input signals to the N availablephysical subcarriers; and an inverse fast fourier transform (IFFT)processor configured to modulate N available physical subcarriers withthe Q interleaved input signals.
 23. The transmitter of claim 22,wherein the subcarrier mapper is configured to assign zeros to N_(FFT)-Nunavailable physical subcarriers to provide N_(FFT) input signals, andwherein the IFFT processor is configured to perform an N_(FFT) pointinverse fourier transform on the N_(FFT) input signals.
 24. A receiverfor a wireless communications system, comprising: a serial to parallelconverter configured to convert a received symbol stream to an N_(FFT)symbol wide parallel received symbol stream; a fast fourier transform(FFT) processor configured to perform an N_(FFT)-point fourier transformon the parallel received symbol stream and to generate received symbolscorresponding to N_(FFT) subcarriers; and a subchannel demapperconfigured to select N symbols of the received symbols corresponding tothe N_(FFT) subcarriers, wherein the N selected symbols correspond to Navailable physical subcarriers out of the N_(FFT) subcarriers, andconfigured to reconstruct a transmit data stream associated with thereceiver from the N selected symbols.
 25. The receiver of claim 24,wherein the N_(FFT) available physical subcarriers comprise orthogonalsubcarriers defined across a frequency band including a plurality ofdiscontiguous available bandwidth segments, and wherein the N availablephysical subcarriers are distributed among the plurality ofdiscontiguous available bandwidth segments.
 26. The receiver of claim25, wherein the subcarrier demapper is further configured to receive atleast one pilot signal from at least one of the plurality of availablephysical subcarriers.
 27. The receiver of claim 26, wherein thesubcarrier demapper is further configured to receive a plurality ofclusters in the discontiguous bandwidth segments, each of the clusterscomprising a plurality contiguous subcarriers, and configured to extractthe at least one pilot signal from an adaptive modulation and coding(AMC) subchannel comprising a plurality of contiguous clusters extendingover a plurality of contiguous symbols.
 28. The receiver of claim 27,wherein the AMC subchannel comprises two clusters over three symbols.29. The receiver of claim 27, wherein the AMC subchannel comprises onecluster over six symbols.
 30. The receiver of claim 27, wherein thesubcarrier demapper is configured to locate pilot subcarriers in an AMCchannel including nine subcarriers per cluster according to theequation: pilot_subs(n, k) = 9n + 3m + 1${{{for}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},\left\lceil \frac{N}{9} \right\rceil,$wherein: n is an index number of a pilot subcarrier in a symbol; k is anindex of the symbol; m is given by k mod 3; and ┌X┘ denotes the largestinteger not greater than X.
 31. The receiver of claim 27, wherein thesubcarrier demapper is further configured to extract a plurality of datasymbols from data subcarriers in the AMC subchannel according to ascrambling sequence.
 32. The receiver of claim 31, wherein thescrambling sequence is defined by a Galois field and an offset.
 33. Thereceiver of claim 31, wherein the scrambling sequence is unique to aparticular cell and/or sector of a wireless communications system. 34.The receiver of claim 24, further comprising: a parallel to serialconverter configured to convert the reconstructed transmit data streamto a parallel communications signal; and a despreader configured todespread the parallel communications signals using a spreading codeassociated with the receiver.
 35. A communications system, comprising: atransmitter comprising: a subcarrier mapper configured to receive aplurality of input symbols, configured to assign the plurality of inputsymbols to N logical subcarriers, configured to map the N logicalsubcarriers to N available physical subcarriers out of N_(FFT) physicalsubcarriers, and configured to generate N_(FFT) transmit symbolscorresponding to the N_(FFT) physical subcarriers; an inverse fastfourier transform (IFFT) processor configured to perform an inversefourier transform on the N_(FFT) transmit symbols output by thesubcarrier mapper; and a parallel to serial converter configured toconvert an output of the IFFT processor to a serial output stream; and areceiver comprising: a serial to parallel converter configured toconvert a received symbol stream to an N_(FFT) symbol wide parallelreceived symbol stream; a fast fourier transform (FFT) processorconfigured to perform an N_(FFT)-point fourier transform on the parallelreceived symbol stream and to generate received symbols corresponding toN_(FFT) subcarriers; and a subchannel demapper configured to select Nsymbols of the received symbols corresponding to the N_(FFT)subcarriers, wherein the N selected symbols correspond to N availablephysical subcarriers out of the N_(FFT) subcarriers, and configured toreconstruct a transmit data stream associated with the receiver from theN selected symbols.
 36. The communications system of claim 35, whereinthe N_(FFT) available physical subcarriers comprise orthogonalsubcarriers defined across a frequency band including a plurality ofdiscontiguous available bandwidth segments, and wherein the N availablephysical subcarriers are distributed among the plurality ofdiscontiguous available bandwidth segments.
 37. The communicationssystem of claim 36, wherein the subcarrier mapper is further configuredto assign at least one pilot signal to at least one of the plurality oflogical subcarriers; and wherein the IFFT processor is furtherconfigured to modulate each of the available physical subcarriers withdata from a corresponding logical subcarrier.
 38. The communicationssystem of claim 37, wherein the subcarrier mapper is further configuredto set an input data signal corresponding to each of the plurality ofunavailable physical subcarriers to zero.
 39. The communications systemof claim 38, wherein the subcarrier mapper is further configured todefine a plurality of clusters in the discontiguous bandwidth segments,each of the clusters comprising a plurality contiguous subcarriers,configured to define an adaptive modulation and coding (AMC) subchannelcomprising a plurality of contiguous clusters extending over a pluralityof contiguous symbols, configured to allocate a first plurality ofsubcarriers within the AMC subchannel as pilot subcarriers, such thatthe pilot subcarriers are distributed uniformly across the AMCsubchannel, and configured to allocate a second plurality of subcarrierswithin the AMC subchannel as data subcarriers.
 40. The communicationssystem of claim 39, wherein the AMC subchannel comprises two clustersover three symbols.
 41. The communications system of claim 39, whereinthe AMC subchannel comprises one cluster over six symbols.
 42. Thecommunications system of claim 39, wherein the subcarrier mapper isconfigured to allocate the pilot subcarriers for an AMC subchannel atlocations determined by the indices of logical subcarriers.
 43. Thecommunications system of claim 39, wherein pilot subcarriers in an AMCchannel are offset by two subcarriers in adjacent symbols.
 44. Thecommunications system of claim 43, wherein the pilot subcarriers for anAMC channel including nine subcarriers per cluster are determinedaccording to the equation: pilot_subs(n, k) = 9n + 3m + 1${{{for}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},\left\lceil \frac{N}{9} \right\rceil,$wherein: n is an index number of a pilot subcarrier in a symbol; k is anindex of the symbol; m is given by k mod 3; and ┌X┘ denotes the largestinteger not greater than X.
 45. The communications system of claim 39,wherein the subcarrier mapper is further configured to assign aplurality of data symbols to data subcarriers in the AMC subchannelaccording to a scrambling sequence.
 46. The communications system ofclaim 45, wherein the scrambling sequence is defined by a Galois fieldand an offset.